Parallel controllable optical fiber positioning system for LAMOST

In present paper we have proposed an optical fiber positioning system for LAMOST (Large Shy Area Multi-Object Fiber Spectroscopy Telescope). In this system, the convex focal plate of the telescope(Its diameter is 1.75m) is divided into about 4000 individual domains. Each domain contains a controllable unit which is named X-unit. Each unit is specified with the position of its domain with polar coordinates (the angle and the radius distance from the center) by using its center point as reference. Each X-unit can be moved smoothly by verifying the two parameters of polar coordinates. An optical fiber is hold with X-unit and introduced to spectroscope, thus the optical fiber can be moved anywhere within its designated circular domain at the convex focal surface. In addition, the circular domains are overlapped in a "honeycomb-like" arrangement to ensure that there are no blind spots on the convex focal surface, such as that the radius distance of the units is 25 mm, and the positioning range of the optical fibers is 30 mm, thus there will be no "blind areas". The structural parameters of the X-unit could also be optimized to limit the directional error of the axis in a certain range. Each unit is driven by two stepping motors and mobilized by computer in a way to ensure that the units do not collide with each other. It is possible that very high or very low star densities in areas of the field of view may reduce the observational efficiency of the telescope. This should not a significant problem by using optimization of the observation program.